The present invention relates to an inline 3-beam color cathode-ray tube electron gun for use as a color image receiving tube or a color cathode-ray tube comprising a color display device and so on.
At present, there is an increasing demand of improving a resolution of a color cathode-ray tube. In particular, a problem concerning a shape of an electron beam spot at the periphery of a picture screen receives a remarkable attention.
In general, a resolution characteristic of a color cathode-ray tube considerably depends upon the size and shape of an electron beam on the fluorescent screen serving as a screen. That is, if the diameter of this electron beam spot were not small and were not close to a real circle, a satisfactory resolution characteristic could not be obtained.
As a deflection angle of an electron beam increases, an electron beam passage ranging from a cathode-ray tube electron gun to a fluorescent screen is extended. Therefore, if a focusing voltage is maintained in order to obtain an electron beam spot of a small diameter and of a real circle at the central portion of the fluorescent screen, the electron beam at the peripheral portion of the fluorescent screen is placed in the so-called over-focusing state. As a consequence, an electron beam spot of a small diameter and of a real circle cannot be obtained at the peripheral portion of the fluorescent screen so that a satisfactory resolution cannot be obtained.
To solve the above-mentioned problem, there is recently proposed a dynamic focusing system cathode-ray tube electron gun in which a main lens action is weakened by increasing a focusing voltage relative to electron beams bombarded on the peripheral portion of the fluorescent screen as the deflection angle of the electron beam increases.
This dynamic focusing system, however, is not so suitable for the inline 3-beam system cathode-ray tube electron gun without modification. That is, in the prior-art inline 3-beam system cathode-ray tube electron gun in which three cathodes are aligned on one linear line in the horizontal direction, when deflection magnetic fields of a deflection yoke are equal, a vertically-arcuate convergence error (i.e. over-convergence) occurs in the upper, lower, right and left peripheral portions of the fluorescent screen.
Accordingly, a dynamic convergence is executed under the condition that a horizontal deflection magnetic field distribution obtained by the deflection yoke is presented as a pin-cushion-like distribution and that a vertical deflection magnetic field distribution is presented as a barrel-like distribution.
However, when the deflection yoke thus arranged is in use, electron beams deflected toward the peripheral portions of the fluorescent screen after they had passed the deflection yoke are subjected to a convergence action (convex lens effect) in the vertical direction (longitudinal direction) thereof and also subjected to a divergence action (concave lens effect) in the horizontal direction (lateral direction) thereof.
As a result, an electron beam spot at the peripheral portions of the fluorescent screen does not become a real circle but becomes oblong. There is then the problem that the electron beam spot is distorted in the left and right peripheral portions of the fluorescent screen and that the focusing characteristic is deteriorated.
In order to solve the aforementioned problems, Japanese laid-open patent publications Nos. 61-99249, 62-237642 or Japanese laid-open patent publication No. 3-93135, etc. proposed cathode-ray tube electron guns having a so-called electrostatic quadruple lens (hereinafter simply referred to as "quadruple lens") incorporated therein.
FIG. 1 is a schematic diagram showing an arrangement of a color cathode-ray tube electron gun incorporating a quadruple lens used widely.
As shown in FIG. 1, an electron gun 70 includes three cathodes KR, KG, KB parallelly arrayed in an inline fashion. A first electrode 11, a second electrode 12, a third electrode 13, a fourth electrode 14, a fifth electrode, a sixth electrode 16 and a shield cup 17 are coaxially disposed from this cathode K (KR, KG, KB) to the anode side, in that order. Then, the fifth electrode is halved to provide a 5-1th electrode 51 and a 5-2th electrode 52. The second electrode 12 and the fourth electrode 14 are connected with each other electrically.
In this color cathode-ray tube electron gun 70, a constant focusing voltage V.sub.F is applied to the 5-1th electrode 51. On the other hand, a voltage (V.sub.F +V.sub.DF) in which a parabolic waveform dynamic focusing voltage VDF (see FIG. 4) synchronized with the horizontal deflection of the focusing voltage V.sub.F and the focusing voltage VF are superimposed upon each other is applied to the third electrode 14 and the 5-2th electrode 52.
Thus, a quadruple lens (not shown) is formed between the 5-1th electrode 51 and the 5-2th electrode 52, and this quadruple lens causes an intensity change of a focusing lens formed between the 5-2th electrode 52 and the sixth electrode 16. As a result, it is possible to obtain satisfactory shapes of electron beams on the left and right peripheral portions of the fluorescent screen.
On the surface of the 5-1th electrode 51 opposing the 5-2th electrode 52 is disposed a plate 151 in which there are defined vertically-oblong electron beam passing apertures 151A, 151B, 151C shown in FIG. 3A. On the other hand, on the surface of the 5-2th electrode 52 opposing the 5-1th electrode 51 is disposed a plate 152 in which there are defined horizontally-oblong electron beam passing apertures 152A, 152B, 152C shown in FIG. 3B.
FIG. 2 is a schematic diagram showing an arrangement of a color cathode-ray tube electron gun incorporating a quadruple lens used widely.
While the fifth electrode is halved to provide the 5-1th electrode 51 and the 5-2th electrode 52 in the electron gun 70 shown in FIG. 1, in an electron gun 80 shown in FIG. 2, the fifth electrode 5 is divided by three to provide the 5-1th electrode 51, the 5-2th electrode 52 and a 5-3th electrode 53 as shown in FIG. 2. A rest of the arrangement of the electron gun 80 is similar to that of the electron gun 70 shown in FIG. 1. Therefore, in FIG. 2, elements and parts identical to those of FIG. 1 are marked with the same reference numerals and need not be described in detail.
In this color cathode-ray tube electron gun 80, as shown in FIG. 2, the constant focusing voltage VF is applied through a stem portion to the central 5-2th electrode 52 of the fifth electrode thus divided by three. On the other hand, the voltage (V.sub.F +V.sub.DF) in which the dynamic focusing voltage VDF (see FIG. 4) synchronized with the horizontal deflection of the focusing voltage VF and the focusing voltage VF are superimposed upon each other is applied to the third electrode 13 and the 5-1th electrode 51 and the 5-3th electrode 53 located in the outside of the fifth electrode thus divided by three.
Thus, two quadruple lenses (not shown) which are adapted to act in the opposite directions, respectively, are formed between the 5-1th electrode 51 and the 5-2th electrode 52 and between the 5-2th electrode 52 and the 5-3th electrode 53. The two quadruple lenses causes an intensity change of a focusing lens (not shown) formed between the 5-3th electrode 53 and the sixth electrode 16. As a result, shapes of electron beams in the left and right peripheral portions of the fluorescent screen may become more satisfactory, i.e. may become substantially close to the shape of the electron beam in the central portion of the fluorescent screen.
On the surface of the 5-1th electrode 51 opposing the 5-2th electrode 52 and the surface of the 5-2th electrode 52 opposing the 5-3th electrode 53 is disposed the plate 151 on which there are defined the vertically-oblong electron beam passing apertures 151A, 151B, 151C as shown in FIG. 3A.
On the other hand, on the surface of the 5-2th electrode opposing the 5-1th electrode and the surface of the 5-3th electrode 53 opposing the 5-2th electrode 52 is disposed the plate 152 in which there are defined the horizontally-oblong electron beam passing apertures 152A, 152B, 152C as shown in FIG. 3B.
Since the quadruple lenses are provided as described above, as the electron beams approach the end portions of the fluorescent screen in the horizontal direction, the electron beam is subjected to a divergence action (concave lens effect) in the vertical direction (longitudinal direction) thereof, and also subjected to a convergence action (convex lens effect) in the horizontal direction (lateral direction) thereof. As a consequence, electron beam spots on the peripheral portions of the fluorescent screen become almost real circles, thereby resulting in a satisfactory resolution being obtained.
The effects achieved by the quadruple lenses are remarkable as described above.
The method of operating the quadruple lens and the dynamic focusing voltage simultaneously is widely available in electron guns for use in color-cathode ray tubes of a display, a jumbo-size TV and a high-definition TV.
In the above-mentioned prior-art technologies, the three electron beams R, G, B receive the quadruple lens effect of the same amount.
Accordingly, as shown in FIG. 5, three electron beams R, G, B emitted from an electron gun 1 and which impinge upon the peripheral portions of the right-hand side screen and the left-hand side screen of a fluorescent screen 4 are converged and diverged in the magnetic field of a deflection yoke 2 by different amounts, respectively, so that distortion states of electron beams on the right and left peripheral portions of the fluorescent screen 4 become different in the three electron beams R, G, B. In FIG. 5, reference numeral 3 denotes a glass bulb. Also, "right-hand side screen" and "left-hand side screen" refer to the right-hand side and the left-hand side presented when a viewer watches the fluorescent screen 4 of the color cathode-ray tube from the outside.
The focusing voltage VF or the like is generally set in such a manner as to optimize the shape of the beam spot of the central electron beam G of the three electron beams R, G, B.
In this case, when the three electron beams R, G, B impinge upon the right-hand side of the fluorescent screen 4, the electron beam R is more strongly affected by the deflection magnetic field formed by the deflection yoke 2 as compared with the electron beams G and B. As a consequence, the distortion of the beam spot of the electron beam R on the fluorescent screen 4 becomes larger than those of the remaining electron beams G, B.
On the other hand, when the three electron beams R, G, B impinge upon the left-hand side of the fluorescent screen 4, the electron beam B is more strongly affected by the deflection magnetic field formed by the deflection yoke 2 as compared with the electron beams G and R. As a result, the distortion of the beam spot of the electron beam B on the fluorescent screen 4 becomes larger than those of the remaining electron beams R, G.
FIGS. 6A and 6B are schematic diagrams showing the manner in which beam spots of electron beams are formed on the fluorescent screen 4, respectively.
FIG. 6A shows the state of the beam spot obtained by the color cathode-ray tube electron gun of the structure having one quadruple lens shown in FIG. 1.
On the other hand, FIG. 6B shows the state of the beam spot obtained by the color cathode-ray tube electron gun of the structure having two quadruple lenses shown in FIG. 6B.
The state (FIG. 6B) of the beam spots of the electron beams obtained in the color cathode-ray tube electron gun of the structure having the two quadruple lenses may provide beam spots of almost real circles and become satisfactory as compared with the state (FIG. 6A) of the beam spots of the electron beams obtained in the color cathode-ray tube electron gum of the structure having one quadruple lens.
However, in FIGS. 6A and 6B, the shapes of the beam spots of the two outside electron beams R, B are different from the shape of the beam spot of the central electron beam G, and deteriorated as compared with the shape of the beam spot of the central electron beam G which is in the so-called just-focus state (i.e. properly focused state).
The innermost beam spot of the three beam spots corresponding to the respective electron beams, i.e. the beam spot of the electron beam R on the right-hand side of the screen and the beam spot of the electron beam B on the left-hand side of the screen are, in particular, deteriorated considerably. In these beam spots, there is presented the over-focused state, and a so-called halation occurs.
In a recent jumbo-size color display monitor having a high resolution, it is frequently observed that red characters on the right-hand side of the fluorescent screen 4 are caused to become unclear by such phenomenon and that blue characters on the left-hand side of the fluorescent screen 4 are caused to become unclear by such phenomenon.
As one means for solving the aforesaid problem, there is known a method of reducing a diameter of a beam spot of an electron beam at the center of the magnetic field by the deflection yoke 2.
Specifically, by reducing the diameter of the beam spot of the electron beam at the center of the magnetic field by the deflection yoke 2, it is possible to reduce the influence exerted upon the electron beams by the magnetic field generated from the deflection yoke 2 depending upon the position at which the electron beams pass the deflection yoke 2 as much as possible.
The above-mentioned means for reducing the diameter of the beam spot of the electron beam, however, encounters with the following problems.
1. Effects achieved thereby are not sufficient: PA1 2. Since the diameter of the beam spot of the electron beam at the center of the magnetic field from the deflection yoke is reduced, the size of the beam spot of the electron beam at the center of the screen is increased.
In a 20-inch color display monitor which is now commercially available on the market, a difference between the focusing voltage VF required when the electron beams R, G, B shown in FIG. 6A are obtained and the focusing voltage VF required when the state of the beam spot of the electron beam R on the right-hand side of the fluorescent screen 4 becomes the state of the beam spit of the electron beam G shown in FIG. 6A amounts to about 100 V.
Naturally, if the state of the beam spot of the electron beam R on the right-hand side end portion of the fluorescent screen 4 is made close to the state of the beam spot of the electron beam G shown in FIG. 6A, then the shape of the beam spot of the electron beam G is deteriorated. Accordingly, the means for reducing the diameter of the beam spot of the electron beam at the center of the magnetic field from the deflection yoke 2 is not effective as the means for solving the aforementioned problem.